Subsurface wells for extracting and/or testing fluid (liquid or gas) samples on land and at sea have been used for many years. Many structures have been developed in an attempt to isolate the fluid from a particular depth in a well so that more accurate testing of the fluid at that depth “below ground surface” (bgs) can be performed. Unfortunately, attempts to accurately and cost-effectively accomplish this objective have been not altogether satisfactory.
For example, typical wells include riser pipes have relatively large diameters, i.e. 2-4 inches, or greater. Many such wells can have depths that extend hundreds or even thousands of feet bgs. In order to accurately remove a fluid sample from a particular target zone within a well, such as a sample at 1,000 feet bgs, typical wells require that the fluid above the target zone be removed at least once in order to obtain a more representative fluid sample from the desired level. From a volumetric standpoint, traditional wet casing volumes of 2-inch and 4-inch monitoring wells are 0.63 liters (630 ml) to 2.5 liters (2,500 ml) per foot, respectively. As an example, to obtain a sample at 1,000 feet bgs, approximately 630 liters to 2,500 liters of fluid must be purged from the well. The time required and costs associated with extracting this fluid from the well can be rather significant.
One method of purging fluid from the well and/or obtaining a fluid sample includes using coaxial gas displacement within the riser pipe of the well. Unfortunately, this method can have several drawbacks. First, gas consumption during pressurization of these types of systems can be relatively substantial because of the relatively large diameter and length of riser pipe that must be pressurized. Second, introducing large volumes of gas into the riser pipe can have adverse effects on the volatile organic compounds (VOC's) being measured in the fluid that is collected. Third, a pressure sensor that may be present within the riser pipe of a typical well is subjected to repeated significant pressure changes from the pressurization of the riser pipe. Over time, this artificially-created range of pressures in the riser pipe can have a negative impact on the accuracy of the pressure measurements from the sensor. Fourth, residual gas pressure can potentially damage one or more sensors and/or alter readings from the sensors once substantially all of the fluid has passed through the sample collection line past the sensors. Fifth, any leaks in the system can cause gas to be forcibly infused into the ground formation, which can influence the results of future sample collections. Moreover, no ASTM standard currently exists for pressurizing plastic riser pipe during the pumping process. Lack of proper standards in this regard can result in uncertainties regarding safe levels of riser pipe pressurization.
Another method for purging fluid from these types of wells includes the use of a bladder pump. Bladder pumps include a bladder that alternatingly fills and empties with a gas to force movement of the fluid within a pump system. However, bladder pumps can be susceptible to leakage due to becoming fatigued or detached during pressurization. Further, the initial cost as well as maintenance and repair of bladder pumps can be relatively expensive. In addition, at certain depths, bladder pumps require an equilibration period during pressurization to decrease the likelihood of damage to or failure of the pump system. This equilibration period can result in a slower overall purging process, which decreases efficiency.
An additional method for purging fluid from a well includes using an electric submersible pump system having an electric motor. This type of system can be susceptible to electrical shorts and/or burning out of the electric motor. Additionally, this type of pump typically uses one or more impellers that can cause pressure differentials (e.g., drops), which can result in VOC loss from the sample being collected. Operation of these types of electric pumps can also raise the temperature of the groundwater, which can also impact VOC loss. Moreover, these pumps can be relatively costly and somewhat more difficult to repair and maintain.
Further, the means for physically isolating a particular zone of the well from the rest of the well can have several shortcomings. For instance, inflatable packers are commonly used to isolate the fluid from a particular zone either above or below the packer. However, these types of packers can be subject to leakage, and can be cumbersome and relatively expensive. In addition, these packers are susceptible to rupturing, which can contaminate the desired fluid sample and result in further significant cost expenditures.